1. Field of the Invention
The invention concerns a method of spraying a powder product and is more particularly concerned with improving the homogeneity of the powder jet. The invention also concerns a powder product sprayer implementing this method.
2. Description of the Prior Art
Powder spraying is a technique used in the most diverse industrial sectors, including: spraying of insecticide, spreading of mineral or organic additives over metal or plastics material parts during manufacture to modify their characteristics, spraying of additives onto the surface of foodstuffs products and spreading of heat-fusible powder paint over objects to be coated prior to passing them through an oven. All these applications, and in particular that last mentioned, require a relatively homogeneous jet of powder. The powder is generally taken from a storage tank in which it is maintained in a fluidized state by a flow of gas, in practise air, fed into the bottom part of the storage tank through a porous bottom wall. A calibrated gas jet fed into a venturi device accelerates and conveys the gas-powder product mixture through several meters of pipe to an ejector or sprayer device, often generally pistol-shaped to facilitate handling. In this type of installation the spraying of powder is sometimes inhomogeneous, which may be due to a number of factors and in particular the concentration of the mixture and the speed at which it circulates through the pipe. Firstly, the flow of the mixture is regular only over a certain range of flow velocities, in the order of 7 through 15 m/s. Below a specific lower speed limit the flow takes place in "packets" of powder. A low-speed flow also favors depositing of powder in the horizontal parts of the pipe and sprayer. Thus when the latter is held substantially horizontal by the user, which is usually the case, powder often accumulates within the casing of the sprayer, which favors inhomogeneous spraying and even partial clogging.
It is nevertheless advantageous to maintain a relatively low flow speed to obtain a relatively wide and slow jet, which reduces wear of the nozzle, avoids "blowing" the powder over the area to be covered and makes it possible to achieve better electrical charging of the particles when the sprayer is of the electrostatic type. Thus it is not possible to increase the speed at which the mixture circulates in the pipe without running into disadvantages. Finally, another cause of "defluidization" of the powder during its transfer from the storage tank to the sprayer (and thus of inhomogeneous spraying) is the centrifugal force created in bends in the pipe. This centrifugal force tends to separate the transporting gas and the powder so that powder accumulates on the outside of all bends. When the powder product sprayer is held horizontally, as already mentioned, the pipe connected to the rear of the sprayer forms a curve in a substantially vertical plane. The powder collects in the upper part of this bend, immediately behind the sprayer, and in many cases inhomogeneous spraying of powder results.
To combat all these phenomena it has already been proposed to place a deflector on the axis of the sprayer orifice, which also has the advantage of widening the powder jet, but the bridges linking this deflector to the end of the sprayer are a further source of inhomogeneous spraying. It has also been proposed to create a "vortex" type turbulent air jet in the vicinity of the sprayer orifice. The functioning of such systems relies primarily on the creation of turbulence in the powder jet. This speeds wear of the nozzle and results in significant head losses which limit the powder flowrate and favor wear.
The invention proposes to rehomogenize the mixture immediately before it is sprayed (that is, in the sprayer itself) by means of a process which does not have the disadvantages mentioned hereinabove.